Ease of transferring weeks / booking floating weeks?

We are considering a floating week resale, but would like to see if anyone has recent experience trying to book a floating week, or transfer a fixed week to another week?

We have family that travels weeks 11 and 12 so on occasion we'd like to book those weeks at a specific island. With only 6 months head time, how hard have you found it to get the fixed week of choice or move a fixed to another week of choice?

Thanks!